I think it's probably just an effect of where the 
cels had been stored before they made it to that 
celbook.  I'm guessing they got a lot of cels from 
the same place, that had been stored pretty 
poorly.  I have seen a few Slayers cels with bad 
line fading, but I've seen the same thing w/ every 
series.  (I would guess Sailor Moon has that 
problem more than anything, not that it will stop 
me from looking for them!!!)  But Slayers is my 
main series to collect and I've seen a whole lot 
more nice ones than faded ones.  So my answer here 
is a definite no, it's not a universal thing with 
Slayers cels.  Look around at some of the 
galleries and you will see.  I know some poeple 
with absolutely stunning Slayers collections.  I 
mean...WOW...  <:)  Good luck in everything!

Kenna...